Pitch circle diameter is a term used to describe the centerline diameter of a circular bolt spacing, the diameter of theoretical friction discs that would transmit the same velocity ratio as two meshing gears, or the diameter at which the thread tooth equals the thread spacing for screw threads. Context should be used to determine whether the term is being used to describe bolt spacing, gears or threads.
For bolt or screw spacing, pitch circle diameter or PCD, also called the bolt circle diameter or BDC, is the diameter of an imaginary circle passing through the center of bolts distributed around a component such as a wheel, hub or housing. Once the pitch circle diameter is established, one need only specify the number of holes or threads and dimensions of each hole or thread to describe the full bolt pattern.
For two meshing gears, the pitch circle diameter represents each of two imaginary friction discs that, when spun together without slippage, would result in the identical velocity ratio and gear ratio as the actual gears. The pitch circle diameter is thus an important geometrical term for engineers in describing the mechanical relationship of a gear set.
To accurately measure the pitch circle diameter or pitch diameter of a thread requires either an optical comparator measuring device or thread wires.